For optical applications in the extreme ultraviolet wavelength range (EUV) particularly in lithography, almost exclusively reflective optical elements are usually used, the surfaces of which desirably satisfy stringent desired properties with regard to figure and roughness. It is known from to produce such reflective optical elements from glass, for example quartz, or from glass ceramic, for example Zerodur or ULE. The surfaces of these elements are usually polished via so-called superpolishing until a microroughness of the surface is less than 0.2 nm rms.
However, these substrate materials are often suitable only to a limited extent for operation with very high thermal loads which can occur at high radiation powers. This can be all the more applicable if the optical element is configured in the form of a gathering collector mirror for an EUV radiation source. In this case, in general, the high radiation power not only has to be emitted in the EUV wavelength range, but can also result from radiation of the EUV source outside the EUV wavelength range. In these cases, a thermal conductivity of the known mirror materials is often too low to conduct away enough heat from the reflective surface, which can lead to severe heating of the mirror and to a functional disturbance as a result of bending and as a result of thermal degradation of a reflection layer of the mirror.
US 2004/051984 A1 discloses, for the purpose of improving cooling of a reflective optical element, applying a cooling medium to a surface. Stress-dictated distortions of the mirror surface are intended to be substantially reduced by this approach. In such an approach, the cooling capacity can be restricted.
US 2007/0091485 discloses a reflective optical element in the form of a mirror for use in an EUV system. The mirror includes a base body divided into two, which is produced from silicon, Zerodur, copper or Invar and which includes an upper part and a lower part. For the purpose of cooling the mirror, cooling channels are arranged in the upper part of the base body, the cooling channels being configured as so-called microchannels having a diameter of 100 μm or less. In this case, the microchannels are designed such that a laminar flow is established in the channels. This system can be complex and expensive to produce.
US 2008/0043321 A1 discloses a further reflective optical element, which is composed of a plurality of mirror segments and can be used in an EUV lithography system. US 2008/0043321 discloses a multiplicity of substrate materials from which the mirror segment can be constructed. A so-called smoothing layer and a reflection layer are applied on the mirror segment. The mirror segments are connected via bonding to form a mirror.
US 2009/0147386 A1 discloses a mirror for EUV applications which includes a base body, on which a reflective surface is arranged. The mirror furthermore includes a heat dissipation device, which is arranged on a rear side of the base body at a distance from the base body. The heat dissipation device and the base body are connected to one another via bending elements. Cooling channels through which a cooling liquid can be passed are arranged in the heat dissipation device. With the aid of the circulating cooling liquid in the cooling channels, the intention is to obtain a defined temperature distribution in the heat dissipation device.